In 1979 the IEEE published an article which has become a basic reference in the field of television echo (or "ghost") elimination. The article is entitled "A Tutorial On Ghost Cancellation In Television Systems" and was written by Walter Ciciora, Gary Sgrignoli and William Thomas and it is incorporated by reference herein.
Although the Ciciora article described the fundamental principles, apparatus and algorithms applicable to ghost cancellation, the state of the art has only recently progressed to the point of providing practical ways to implement and improve these basic concepts.
One of the above referenced U.S. Pat. No. 5,034,907, provides an example of a signal processor architecture which can be programmed to perform varied filter and other complex digital processing operations utilizing programmed sequences of coefficients and control words. The other referenced U.S. Pat. No. 5,161,017 describes an echo cancellation apparatus which can effectively be implemented with the instant invention.
There are two main steps to the echo cancellation process. First the characteristics of the communications channel (which include the echo artifacts, if any) must be determined at the receiver. Once the channel characteristics are calculated, filters are used to implement the inverse channel to perform the echo cancellation. Because the channel characteristics may include more than one type of echo, each of which is preferably processed differently, a need exists for a circuit architecture, which among other things, provides the ability to programmably optimize a desired filter architecture for the derived channel characteristics with the speed necessary to be effective in a real time communications system. It is therefore an object of the invention, to provide a circuit architecture which lends itself to rapid filter array configuration under programmed control.
A received video signal contains echoes which are comprised of superimposed copies of the originally transmitted signal, which have different delay times and amplitudes. The strongest signal component represents the originally transmitted, or "main" signal component. Looking in the time domain, any copy component occurring before the main signal component is called a "pre-echo" component and any copy components occurring after the main signal component is called a "post-echo" component.
An IIR filter is inherently causal in nature and therefore cannot be used to cancel pre echoes. An IIR filter can, however, effectively be used to substantially cancel post-echoes.
An FIR filter, can be both causal and non-causal. The non-causality allows cancellation of pre-echoes. An FIR, filter however, cannot perform ideally unless it is of infinite length. A practical (i.e. finite length) FIR filter will therefore only suppress, but not completely cancel, the pre-echoes. The longer the FIR filter however, the better the pre-echo will be suppressed. A practical echo cancellation circuit should include therefore, an FIR filter to suppress the pre-echoes followed by an IIR filter to suppress the remaining post echoes. It is therefore another object of the invention to provide an echo cancellation circuit which includes both IIR and FIR filters for effective cancellation of both post-echoes and pre-echoes.
The bandwidth of a standard television channel, for example an NTSC channel, is about 4.2 MHz. Any digital processing therefore has to be done at a rate of 8.4 MHz. or more in order to meet the Nyquist sampling criterion. Often the processing of composite baseband NTSC signals is done at a 14.32 MHz. rate, which is four times the frequency of the color subcarrier. All echoes, for the most part, fall in a range of -3.5 us. to +45 us. with respect to the main signal component, in the time domain. For third order cancellation of pre-echoes therefore, the FIR should span about 10.5 us. and the IIR should span about 45 us. At the 14.32 MHz. sample rate for example, this would require the FIR filter to have 150 taps and the IIR filter 645 taps. These would be large and expensive filters. Fortunately, there is a tendency for the dispersed echo components to "cluster" and this property permits the design and use of filters (known as "sparse" filters) which do not require filters at every tap. Since echoes are dispersive, several taps are required to cancel them. Echoes with a phase shift in the RF domain, and echoes that do not exactly coincide with sample moments, also require multiple taps to be cancelled effectively. Taking advantage of the tendency of the dispersed echoes to cluster, each echo can be cancelled by a cluster of filter taps. Is is therefore another object of the invention to provide an architecture suitable for the implementation of sparse filters.